Prusa i3 Z Axis Binding: 7 Common Issues & Solution

The Prusa i3 Mk3S+ is viewed as the best option at the price. But the printer is not without issues, among which is the Z-axis binding, resulting in the Z-axis skipping. 

So this article is written to introduce the main causes for Z-axis skipping and sticking — loose fixings, poor alignment, bent lead-screws, tired motors or drive electronics, faulty wiring etc. — and to lead you through a diagnostic process that will help you to learn more about the best function of your machine, identify and fix issues that arise and know how to stop them from happening again.

What Is Z-Axis Binding?

The Z-axis has two drive screws that work together to lift/lower the gantry.

One of the strengths of the i3 is that the Z-axis drives are independent, each with its own stepper motor — but they’re not really independent, because the X-axis gantry has to remain level, so these motors must drive step by step.

And there’s the problem. If one of the drives stalls, or simply loses a few steps every move, soon that X gantry is skewed leading to the Z-axis not moving.

So Z-axis binding happens when these two drive motors get out of step, because stray forces exceed its drive motor step torque.

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7 Reasons Why Prusa i3 Z-Axis Is Slipping

There are a number of reasons why a thread may drag, a bearing bind, or a drive mechanism stiffen — and they produce similar symptoms, making diagnosis less than simple.

Every effect has a cause, and with patience, you can find what’s wrong. And usually, it’s not a replace/repair issue but a setup that needed more delicacy in assembly, or dirt or wears/damage.

1. Loose Eccentric Nuts

One of the simplest causes of Z-axis slippage is the eccentric nuts that lock the Z carriage to the vertical guide bar being loose.

If the carriage is not correctly aligned by these, the axis can tighten up and slip on one side during Z motion. This can cause binding that exceeds the motor’s step force, so steps get missed.

It may also pull the whole carriage out of alignment and it binds up, which you discover mid-print!

2. Debris In Or Bruising Of The Threads

Threaded lead–screws are a great way to transmit drive, converting motor rotation into back-lash free linear motion.

But the drives rely on a precise relationship between the lead-screw and the trapezoidal nuts that should run up and down them smoothly.

Get dirt or printer debris in there and that relationship isn’t so sweet, and a pulsating drag will result.

Equally, if there’s damage to either the internal thread of the nut, or to the lead screw, there’ll be tight spots or general binding — and if that drag force exceeds the stepper motors torque, steps will be lost and misalignment will result.

3. Bent Or Misaligned Lead-Screws

If the lead screws are bent, or they splay outwards at the top or bottom, the thread motion will get tight somewhere in its travel.

The lead screws are never perfectly straight, and since their top and bottom ends are constrained, the bends will transfer load onto the X-axis.

One way to know your lead screws are less than straight is if you get banding in the Z direction that’s at the same pitch as the screw.

That’s a small X offset caused by the Z lead-screw transferring some of the load from the gantry fighting to straighten the screws.

4. Incorrect Linear Bearing Binding Ratio

The X-axis is carried on two recirculating—ball linear bearings on each side, running on the guide rods. These are locked by a set screw and the X gantry should slide freely from top to bottom.

There are two ways that this mechanism can become sticky because of alignment forces on one side, or both:

• If the two Z drives are out of step, then the X gantry is being forced into an S shape, one end higher than the other — which will likely cause the slide to bind over the whole Z travel.
• If the two guide rails are out of parallel, or not straight, then the slide will tighten up where the gap between rails is larger or smaller than the bearings can accommodate, causing the balls to stiffen. Tight at the top or bottom suggests alignment, a tight spot, and mid-motion suggests bending.

5. Lubrication Issues

There are four running surfaces that guide and drive the Z gantry — two recirculating—ball slides running on guide rails on each side; and one trapezoidal nut on the lead screw on each side.

A heavily used machine can get a couple of issues that disrupt these elements — either loss of lubricant, or contamination making the lubricant lumpy.

The lead screws are unprotected from dirt and barely lubricated, so they dry out and get contaminated.

The slides have a better time in lubrication — wipers should clean off detritus but they wear and get damaged. And once those tracks get dirt in them, it’s hard to get out.

6. Missed Steps When Nothing Seems Wrong

Your stepper motors are driven by some smart electronics — and sometimes this is not quite as smart as it seems.

If you’re losing alignment and binding on the Z–axis, there’s a motor setting you can adjust — once you’ve ruled out all the other variables and done the maintenance that should solve the problem.

VREF is the reference voltage setting in the stepper motor variables, used to adjust the step current. When you increase VREF, you increase the stepper motor torque by increasing the step drive current.

Of course, there’s no such thing as a free lunch. Increased current means increased temperature in the motor and the driver board — which has the potential for bad consequences.

So tread carefully and be ready for new problems if you overdo this! But equally, this may highlight deeper issues such as tired motors, failing drivers, or faulty wiring.

7. Feed Rate For Z-Axis

One fact guides any complex machine operation — and the lighter and more flexible the machine, the more the fact applies.

Speed kills. If you run your axes at high speed, any small issues like those above will be amplified.

The Z–axis motors are more likely to lose steps in fast motion — and as a rule, doubling the axis step rate will give you four times the risk of step errors, resulting in loss of alignment and binding.

Any small binding forces that your machine can cope with fine at lower speeds may become aggravated by high speed, so your desire for faster print jobs may be the straw that seems to upset this particular camel.

But the feed rate is likely not the issue — it’s the canary in your coal mine that’s telling you there’s a screw, bearing, alignment, lube, or motor/VREF issue you should look at.

Fix what the feed rate is triggering to fail, and you’ll make your printer better.

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How To Fix Prusa i3 Z-Axis Binding

Let’s work through the list, because there are solutions, or ways to evaluate if you have, and how you can cure each possible failure mode.

You need to follow a diagnostic process, to start ruling out variables. However, this is a learning process and you’ll need to feel your way — there’s no simple flow chart for this, but there are clear steps and logical order:

1. Evaluate For Loose Components

First, you need to evaluate whether anything is loose. Look at the eccentric nuts that locate the Z-axis bearing assemblies.

Have they come loose? If so, the safest approach is to loosen both, run the axis to zero and then tighten them again.

Is anything else loose? If yes, then carefully tighten it.

2. Check For Electrical Problems

If there’s nothing loose or out of place, the next step is electrical problems. Can you identify which side is out of step? When you run the Z-axis from zero to most of the way up, one side may be lagging – that’s the side with a fault.

Here, the machine design is your friend, because the two stepper motors should get identical signals.

If this is an electrical fault, then the problem will swap sides when you swap the connectors for the Z stepper motors and run the axis to zero and then all the way up.

If the problem stays on the same side, it’s ‘mechanical’; if it swaps sides, it’s ‘electrical’.

If it’s electrical, check the wiring. A broken or poor connection can be intermittent and cause random or regular missed steps. With the machine powered down, check continuity with a meter or a buzzer, where you can.

If the fault looks like it’s electrical and the wiring is all good, restore the connectors and swap the motors. If the ‘electrical’ problem swaps sides, you have a tired motor. If it stays put, you may have a tired driver board.

This is not a deep analysis; it will only find gross faults and narrow down the things to replace – but that can save you a lot of money and disappointment.

3. Find Lead-Screw and Linear Bearing Problems

Once you’ve ruled out loose and broken stuff, and electrical issues, you need to try to identify which part(s) are playing up. Do this with as little take-apart as practical.

The aim is to isolate the problem in a sub-assembly, or rule out problems by freeing up the motion and ‘feeling’ for the tight spots

Drive the axis lead screws through their whole motion while gently touching the thread — not hard enough to add drag and make things worse.

You’re feeling for reluctant or tight spots, hoping to feel the missed steps. This will tell you if you have a problem over the whole range of motion or just at one spot and it may tell you which side (if only one) is tightening.

If you think you can feel or see the tightness in one side, it might be the lead-screw and it might be the linear bearings, so first decide if:

• It’s tight all the way through the travel — then there’s some stuff stuck in the trapezoidal nut, or the nut thread is damaged or worn.
  • Remove the Z-axis top-part from the problem side and unscrew the trapezoidal nut retaining screws and wind the nut up and off the lead screw. If it runs tightly or with pulsing tightness, that’s your problem. If not, it’s likely the linear bearing so put the trapezoidal nut back.
  • Examine inside the threads, and look for burrs, bruising, or material scraps. Damaged threads can often be eased to reduce the drag — and trash can be removed. If there’s obvious wear, replace the nut. Run the repaired/cleaned/new trapezoidal nut down the lead-screw to confirm the problem is reduced.
  • Reassemble and test.

• There’s pulsing tightness through the motion, once per screw revolution — then your lead-screw on one side (or both) isn’t straight enough. You’ll need to replace them to solve this — but first remove the screw and roll it on a flat surface — glass is ideal. Use a paper shim to test the tightness, if you cannot see the bend in the rolling.
• There’s pulsing tightness that’s less frequent than once per turn of the screw — in which case it’s likely the linear bearings that are the problem. Disassemble and clean the linear slides, relubricate and reassemble. When the bearings are removed, slide them on the shaft — if there is a tight spot each bearing length of movement, then a ball is likely damaged or there’s dirt in the track. Clean and lubricate or replace, and if you’re reusing cleaned bearings, make sure the wipers are not worn or damaged.
• There’s tightness at the top or bottom of the motion — which suggests the vertical rods are not parallel (easy to fix by resetting them) or bent (easy to fix by replacing them). 

Reassemble with the mountings at both ends loose, move the axis to the bottom, tighten the bottom mounts, travel Z to the top, and repeat.

That should realign the guide rods and reduce the drag.

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Conclusion

The Prusa i3 is a simple, effective, and easy-to-diagnose machine. Learn your way around, change the setup and see what happens. Read forums and watch user videos. You can fix this!